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<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>

let a = 100;
let b = a;
a = 200;
console.log(b); //100

let c = {age:20};
let d = c;
c.age = 21;
console.log(d.age); //21
    </script>
</body>
</html>